The use of electronic product code (EPC) tags is expanding on a global basis. EPC tag cost is dropping, and EPC performance continues to periodically improve. Passive EPC tags acquire all required energy from an RF identification impulse signal sent by a reader module (“control unit”), while battery-assisted tags, such as battery-assisted tags from Goliath Solutions, LLC, utilize energy from an attached battery in generating a response to a received identification impulse signal. Identification impulse energy declines with the square of the distance from the reader module. Physics of tag sensitivity and strict regulations regarding maximum reader/antenna power output combine to produce limits as to the distance a passive EPC or battery-assisted tag can be located and read from a given antenna.
FIG. 1 depicts an example plot of the received power of an identification impulse signal at a passive EPC tag versus the distance of the tag from the reader module. A tag threshold is included at −18 dBm as a dashed line. The tag threshold sensitivity is a key determinant of the maximum distance from the reader that a passive EPC tag may be read. Under current FCC regulations and technology, the maximum distance from a reader that a passive EPC may be read is about 12.5 meters under very good conditions. Battery-assisted tags may be effective at further distances because the transmitted identification impulse signal need only reach the tag with enough power to be read by the tag, as the response may be assisted by the attached battery. Despite the added coverage distance, which may be several times the effective passive EPC distance, many choose to utilize the passive EPC tags due to their reduced cost and size, as well as their potential for use with other applications that rely on standard reader protocols.
EPC RF interference with and from other RF systems has also been a substantial difficulty in EPC system design. Lighting, cell phones, inventory scanner guns, and even nearby EPC readers and tags have been found to diminish EPC system performance. This is in addition to environmental challenges, such as the presence of metal or liquid. Because of variances in the size and layout of different locations where RFID systems are to be implemented, which may result in very different or continually changing RF environments in which to operate, expensive RF experts and RF monitoring equipment has often been required in implementing a cost-effective EPC tracking system that is able to support a high enough accuracy rate to be worth the costs of implementing and maintaining.
To combat the high costs of EPC tracking system implementation, the scope and goals of tracking systems are often simplified to mitigate the above-described RF environment difficulties. For example, most EPC applications to date have been limited to a relatively small number of readers, such as at distribution “pinch-points” (e.g., loading docks), where EPC tags in cases and pallets passing through the pinch-point may be monitored. This type of system design limits the space between reader antennas and tags through deliberate placement of readers and antennas along a small number of predetermined paths of tag travel.
FIG. 2 depicts a prior art EPC RFID reader unit. The reader unit 202 includes several RF ports 204 that are dedicated transmit or receive ports. The ports 204 may also be full duplex RF ports, where the reader unit 202 may transmit and receive simultaneously on the same port. While such a system may be effective in monitoring EPC tags in a limited scope, such as the pinch-point monitoring described above, such a system may not be cost effective in broader area RFID monitoring, where a large number of expensive reader units 202 would be required to cover the desired area (e.g., a 10,000-square-foot drug store may require 40 or more reader units 202 to implement full passive EPC tag monitoring coverage). Classically-developed EPC reader and antenna technology is constrained by the distance a given antenna can be from the reader before cable loss attenuates the signal below useable levels. This adds to the cost of the initial system, as well as to the cost of installation due to the number of independent systems that must be deployed to cover a sizable space.